1. Field of the Invention
The present invention relates to a viable power optical system used in an onboard camera, a surveillance camera using an imaging element, such as a CCD and a CMOS, and, more particularly, to a variable power optical system suitable for use in a surveillance camera.
2. Description of the Related Art
A variable power optical system employed in a varifocal lens for use with a surveillance camera provides convenience which allows a user to set a focal length and a distance to a subject according to a user environment, and hence a market for the variable power optical system has recently undergone particular expansion as an optical system for use with a surveillance camera. Meanwhile, a demand for development of a low-cost, high-performance lens system is also escalating.
A surveillance camera primarily intended for surveillance use is required to be able to cover a wide angular field of view to a standard field angle and withstand indoor or outdoor use. Therefore, a negative-positive two-group zoom lens that has a comparatively simple structure and that conforms to conditions to a certain extent is heavily used. In addition to having an advantage of simple structure, the negative-positive two-group zoom lens has other advantages. Namely, a negative group is positioned in front of a positive group, whereby a wide field angle is acquired; and a long back focal length can be acquired for a short focal length. Therefore, the negative-positive two-group zoom lens is widely used.
From the viewpoint of the above-mentioned intended use, lens specifications of the variable power optical system require a large diameter fast lens system including an ultra wide angle field. Therefore, in order to maintain a certain degree of performance, a related-art spherical lens system requires a greater number of constituent lenses, and the lens is prone to being greater in diameter and cost.
Accordingly, in order to curtail cost by means of the current technology, it is necessary to reduce the number of constituent lenses and heavily use a plastic material that is inexpensive in terms of a material cost.
Variable power optical systems described in; for instance, JP-A-2008-112000, JP-A-01-303409, JP-A-2001-281544, and JP-A-2006-251437 have already been known as a negative-positive two-group zoom variable power optical system including plastic lenses, such as those mentioned above.
However, many technical drawbacks are encountered when plastic lenses are utilized in the field of a surveillance camera. The drawbacks include the followings. Namely, it is necessary to make a plastic lens less prone to a change in imaging position or performance, which would be caused by environmental changes; in particular, a temperature change, a humidity change, and the like. Moreover, an optical plastic material is very restrictive and has a degree of freedom that is smaller than that of a glass material, which poses a restriction on the design of a lens. Introduction of an aspherical surface technique is indispensable for attempting to achieve higher performance while the number of lenses is reduced.
Moreover, in the field of a surveillance camera, it is required to achieve lens specifications by means of which high speed achieved at F/1.4 is realized while an ultra wide area of 100 degrees or more is covered. However, it has been difficult to implement a variable power optical system having such high specifications by means of the plastic lenses described in JP-A-2008-112000, JP-A-01-303409, and JP-A-2001-281544. Further, although the plastic lens described in JP-A-2006-251437 corresponding to US 2006/203355 fulfill the target high specifications, the lenses include glass aspherical lenses that are costly to manufacture and cannot be said to be less expensive.
A digital camera and a movie camera have a built-in automatic focusing function and therefore can automatically make corrections to positional displacements of an image. However, in the field of a surveillance camera, it is preferable to compensate for positional displacement of an image by use of only a lens system so that the lens system can be attached to a camera not having an automatic focusing function.